Integrated circuits are electrical systems comprising solid-state switches (transistors) connected by thin-film conductive traces. Heat is generated by large numbers of transistors in a small area, switching at high frequency. High-frequency switching is a major factor in the generation of heat, because the absolute current flow is proportional to frequency.
There has long been a trend to higher and higher density in IC development, and this trend contributes to heat generation as well. Another factor is associated with use and placement of ICs in circuitry. The high density motivation extends to higher-level circuitry, such as printed circuit boards. In addition, there has long been a trend in the industry to smaller and smaller products, such as the development of laptop computers after desktop units, then notebook computers, then palmtop computers, and recently even smaller units called personal digital assistants.
All of the developments described above lead to increasing difficulty in dissipating the heat generated from IC operation. If heat generated is not disposed of, temperature rises, and if a balance is not reached between heat generation and heat dissipation, temperature may rise to a point where performance is degraded, and even to a point where physical damage may occur. The problems of heat generation and resulting temperature rise are compounded by the fact that, for most materials, resistance increases with temperature.
The problems of heat generation and resulting temperature increase described above apply in particular to microprocessors, and certain characteristics of such temperature problems, though not limited to microprocessors, can be effectively demonstrated and addressed through reference to microprocessors.
FIG. 1 is a somewhat simplified block diagram of a microprocessor comprising several functional units. There are an address unit AU), an execution unit (EU), a bus communication unit BU), and an instruction unit (IU), all connected through address, data, and control buses. This functional-unit architecture is typical of microprocessors, and state-of-the-art microprocessors are generally more complex than that shown in FIG. 1.
Functional units in a microprocessor are typically not used equally. For example, a math-intensive application uses the computational functional unit or units more than other functional units in the microprocessor. As another example, some applications are more memory intensive, or may use logic units to a greater extent. As a result of this unequal utilization, some regions of a CPU generate heat, and therefore tend to increase in temperature, faster than other regions.
Unequal use of regions of a microprocessor can produce hot-spots greatly influencing mechanical stresses in an IC die. ICs are typically manufactured by techniques of layering and selective removal of different materials, so uneven heating may create stresses and flexure because of differing thermal expansion rates for the different materials. The induced stresses and movement can result in micro cracking and fatigue failure.
What is needed is a system implemented on ICs, such as microprocessors, for managing power dissipation to maintain acceptable levels of ICs performance and structural integrity.